A flight plan is generally determined by a flight management system, commonly referred to as FMS. The FMS are installed these days on most civilian aircraft in order to assist the pilots in navigation. A flight plan is notably defined from departure and arrival points and a navigation database. It comprises a chronological sequence of waypoints described by their three-dimensional position and, possibly, a setpoint of altitude to be maintained, of speed to be maintained and/or of overflight time to be maintained. From the flight plan, the navigation database and a performance database of the aircraft, the FMS can determine a three-dimensional trajectory and a speed profile to be followed by the aircraft. The three-dimensional trajectory is formed by a series of segments linking the waypoints in pairs. The projection of the three-dimensional trajectory in a horizontal plane is called lateral trajectory and the projection of the three-dimensional trajectory in a vertical plane is called vertical trajectory or vertical profile. In practice, the lateral and vertical trajectories are often computed independently of one another. The lateral trajectory is computed initially as a function of the list of the waypoints in the flight plan. The vertical trajectory is then computed as a function of the lateral trajectory and of the altitude and speed conditions imposed by the flight plan and by the performance levels of the aircraft. Since the lateral and vertical trajectories are dependent (the turn radii of the curve segments of the lateral trajectory are a function of the ground speed predicted at the point by the vertical trajectory), the current systems perform a certain number of loopbacks to ensure the convergence of the 3D trajectory.
The three-dimensional trajectory of the aircraft is usually optimized in order to reduce the costs generated by the flight. These are notably costs linked to fuel consumption, the activity of the navigating personnel and the maintenance of the aircraft. In practice, the lateral trajectory is determined to offer the shortest possible distance between the departure and arrival points. For various reasons, for example because of the weather conditions along the trajectory, or the detection of a conflict with the trajectory of another aircraft, or else because of a procedure imposed in areas outside of radar coverage, provision is made to be able to offset the lateral trajectory by a certain distance, in one direction or in the other. This offset is commonly called “lateral offset” in the literature. At the present time, it is known practice to define a lateral offset in two different ways. The first type of lateral offset is called “max possible offset”. It consists in constructing an offset trajectory starting from the current position of the aircraft, and continuing to the final waypoint that can be offset. Typically, the trajectory of an aircraft can be offset laterally as far as the landing runway approach phase. The second type of offset is called “offset from A to B”. For this type of offset, the lateral trajectory is offset between a first waypoint or the current position of the aircraft, and a second waypoint, situated after the first point concerned. Each type of offset is defined by four parameters, namely the points of entry and exit of the offset trajectory, the distance between the initial trajectory and the offset trajectory, and the direction (right or left) in which the trajectory is offset.
As they are currently defined, the two types of offset do not allow for an accurate adjustment of the position of the offset trajectory in relation to the initial trajectory, that is to say of the point of entry and of the point of exit of the offset trajectory. Now, in certain situations, for example for long-haul flights, the consecutive waypoints may be relatively distant from one another, so that the pilot of the aircraft may be constrained to divert the aircraft from its initial trajectory over a much longer portion than that where the obstacle to be avoided is located. Furthermore, an offset of the lateral trajectory may be desired for reasons other than avoiding an obstacle. In particular, it may be necessary to fly along an offset trajectory in order to delay the time of arrival at a waypoint or at the landing runway, for example in the case of significant air traffic at a waypoint. In such a case, the parameters currently used do not make it possible to directly define the offset that makes it possible to obtain the desired delay duration.